Biogas is a mixture of around 60% methane, 40% carbon dioxide and traces of other contaminant gases. The exact composition of biogas depends on the type of feedstock being digested.
Biogas can be combusted to provide heat, electricity or both. Alternatively, the biogas can be 'upgraded' to pure methane, often called biomethane, by removing other gases. This pure stream of biomethane can then be injected into the main gas grid or used as a road fuel. One cubic metre of biogas at 60% methane content converts to 6.7 kWh energy.
Biogas can be combusted to produce heat alone. Some of this energy can be used on site to maintain the temperature of the digester and to heat nearby buildings. However, even small plants will have an excess of heat. The heat can be transferred via hot water to remote users by a district heating system, a concept widely used in some European countries like Denmark, or more likely in the UK used by horticultural and industrial businesses in the vicinity.
Using biogas for heating requires investment in new infrastructure, and installations can benefit from the Renewable Heat Incentive (RHI) support. The RHI includes support for biogas combustion and for biomethane injection into the national grid at all scales.
Electricity generation is a relatively straightforward use for biogas and it can be the most profitable. Biogas requires minimal investment in cleaning and upgrading, is easier to transport than heat and supply is easily measured.
Electricity storage, however, is not simple and connecting to the electricity network is costly. The Renewable Energy Association produced an easy to read guide to grid connection including alternatives that may improve the viability of a project. Also see this report by NNFCC and The Andersons Centre for more information on how to sell your electricity.
Combined Heat and Power (CHP)
Combined heat and power (CHP) is the simultaneous production of useable heat and electricity. As the process of AD requires some heat it is suited to CHP and this is currently the most popular option for UK plants. Whilst coal and gas-fired power stations have an efficiency of around 34% and 55% respectively, CHP plants can achieve overall efficiencies in excess of 80% at the point of use.
The ratio of heat to power varies dependent on the scale and technology, but typically 35-40% is converted to electricity, 40-45% to heat and the balance lost as inefficiencies at various stages of the process. This typically equates to over 2kWh electricity and 2.5kWh heat per cubic metre, at 60% methane.
If you plan to export electricity produced by an anaerobic digester to the national grid you will need a Power Purchase Agreement (PPA). A PPA is a legal contract between an electricity generator and an electricity supplier, typically a utility company.
A generator producing electricity from a biogas plant can be connected to a transmission network, distribution system or even to the wires owned by the end customer. A Distribution Network Operator (DNO) owns the wires in the national grid and are the party you will need to deal with regarding your physical connection. The Renewable Energy Association has a useful guide to Connecting Anaerobic Digestion Generators to Distribution Networks in Great Britain. You can identify your DNO and find their contact details here.
In Northern Ireland, the Northern Ireland Electricity (NIE) Transport and Distribution Group are responsible for assessing the feasibility of connecting an AD plant to the grid and the associated costs. An initial feasibility study can be carried out, and then following a successful planning application, a grid study will assess the appropriate connection requirements and the fee involved. If upgrades to the local network are involved, this may require NIE to obtain planning permission which may add a time delay to the process.
Biogas can be upgraded to biomethane and injected into a gas grid. This can be the national high pressure gas transmission grid or a local low pressure gas distribution network. To be used in the gas grid in the UK biogas needs to cleaned of impurities, dried and upgraded to a higher methane content (> 95%) so that it resembles the qualities of natural gas.
Northern Gas Networks have produced a dedicated Biomethane website which includes guidance on the connection process. If you are considering upgrading biogas to biomethane and injecting into the national gas grid you initially need to identify your Gas Distribution Network (GDN) operator and place an enquiry for initial feedback – a GDN map is available here. OFGEM have also produced a factsheet providing basic information on how to get connected and issues arising from biomethane injection.
Injecting biomethane into the grid has several advantages:
- biomethane injection is supported under the RHI
- provides a far more flexible fuel than biogas
- biomethane has a higher energy density than biogas
- ensures energy that is captured in the biogas is used efficiently
However, there are currently many barriers to grid injection, both practical and financial:
- no specific UK standard for biomethane
- upgrading adds substantially to the cost and energy requirement
- upgrading can reduce carbon savings
- no incentive for grid operators to accept biomethane
- AD plants may be some distance from the gas distribution network.
These issues were the focus of the Energy Market Issues from Biomethane (EMIB) working group. The group's report and minutes from previous meetings are available on their website.
A new area of interest is virtual gas pipelines, whereby tankers transport gas to a centralised upgrading facility and connection point. A trailer filled with CNG can transport around 5 tonnes of gas, an LNG tanker around 21 tonnes.
The Quality Protocol for Biomethane from Waste sets out end of waste criteria for the production and use of biomethane arising from the degradation of organic wastes in a landfill site or anaerobic digestion plant, for injection into the gas grid or use in an appliance suitably designed and operated for natural gas. If these criteria are met, the biomethane will normally be regarded as having been fully recovered and to have ceased to be waste.
Biomethane for Transport
Biogas can be cleaned to remove impurities and upgraded to pure biomethane. It can then be used as a renewable transport fuel in vehicles designed to run on compressed natural gas (CNG) or liquefied natural gas (LNG). Biomethane is eligible for support under the Renewable Transport Fuel Obligation. The greenhouse gas savings of biomethane fuelled vehicles can be significant. Methane fuelled vehicles can also have extremely low emissions of local pollutants, including NOx and particulates (PM2.5 and PM10) when compared to modern petrol and diesel vehicles.
NNFCC has produced a calculator to compare financial support for biomethane provided by the RTFO and the RHI.